1. Field of the Invention
The present invention relates to a clamping mechanism that secures a workpiece to a mechanical arm. More particularly, the present invention relates to a clamp that gently secures and centers a semiconductor wafer to a robot blade when the robot arms are at least partially retracted for rotation within a transfer chamber.
2. Background of the Related Art
Modern semiconductor processing systems include cluster tools that integrate a number of process chambers together in order to perform several sequential processing steps without removing the substrate from the highly controlled processing environment. These chambers may include, for example, degas chambers, substrate pre-conditioning chambers, cooldown chambers, transfer chambers, chemical vapor deposition chambers, physical vapor deposition chambers, and etch chambers. The combination of chambers in a cluster tool, as well as the operating conditions and parameters under which those chambers are run, are selected to fabricate specific structures using a specific process recipe and process flow.
Once the cluster tool has been set up with a desired set of chambers and auxiliary equipment for performing certain process steps, the cluster tool will typically process a large number of substrates by continuously passing them, one by one, through a series of chambers or process steps. The process recipes and sequences will typically be programmed into a microprocessor controller that will direct, control and monitor the processing of each substrate through the cluster tool. Once an entire cassette of wafers has been successfully processed through the cluster tool, the cassette may be passed to yet another cluster tool or stand alone tool, such as a chemical mechanical polisher, for further processing.
The amount of time required by each process and handling step has a direct impact on the throughput of substrates per unit of time. Processes that require greater amounts of time to achieve the desired result may necessitate that multiple chambers or tools be operated in parallel. On the other hand, processes that are completed in a short amount of time may be allowed to sit idle for brief periods, depending upon the economic considerations of cost of ownership and cost of operation. However, while the exact design of an integrated circuit fabrication system may be complex, it is almost always beneficial to perform each step as quickly as possible to maximize overall throughput without detrimentally effecting product quality, operating costs or the life of the equipment. One exemplary fabrication system is the cluster tool, shown in FIG. 1, disclosed in U.S. Pat. No. 5,186,718, entitled "Staged-Vacuum Wafer Processing System and Method," Tepman et al., issued on Feb. 16, 1993, which is hereby incorporated by reference.
The substrate throughput in a cluster tool can be improved by increasing the speed of the wafer handling robot positioned in each transfer chamber. The robot of FIG. 1 is shown in greater detail in FIG. 2 and is the subject of U.S. Pat. No. 5,469,035 entitled "Two-axis Magnetically Coupled Robot," issued on Nov. 21, 1995, which is hereby incorporated by reference. Referring to FIG. 2, the magnetically coupled robot comprises a frog-leg type connection or arms between the magnetic clamps and the wafer blade to provide both radial and rotational movement of the robot blade within a fixed plane. Radial and rotational movements can be coordinated or combined in order to pickup, transfer and deliver substrates from one location within the cluster tool to another, such as from one chamber to an adjacent chamber.
As the robot speed and acceleration increase, the amount of time spent handling each substrate and delivering each substrate to its next destination is decreased. However, the desire for speed must be balanced against the possibility of damaging the substrate or the films formed thereon. If a robot moves the wafer blade too abruptly, or rotates the wafer blade too fast, then the wafer may slide off the blade, potentially damaging both the wafer and the chamber or robot.
The robot blade is typically made with a wafer bridge on the distal end of the wafer blade that extends upward to restrain the wafer from slipping over the end. However, the wafer bridge does not extend around the sides of the blade and does very little to prevent the wafer from slipping laterally (i.e., side to side) on the blade. Furthermore, the wafers are not always perfectly positioned against the bridge. Sudden movement or high rotational speeds may throw the wafer against the bridge and cause damage to the wafer or cause the wafer to slip over the bridge and/or off of the blade.
There is a certain amount of friction that exists between the bottom surface of a wafer and the top surface of the wafer blade to resist slippage of the wafer. However, the bottom surface of a silicon wafer is very smooth and has a low coefficient of friction with the wafer blade, which is typically made of stainless steel or ceramic. Furthermore, a typical wafer is so lightweight that the total resistance due to friction is easily exceeded by the centrifugal forces applied during rapid rotation of the robot, even when the blade is in the fully retracted position. However, this low coefficient of friction is typically relied upon when determining the speed at which the robot rotates.
Therefore, there is still a need for a robot that can transfer wafers at increased speeds and accelerations/decelerations. More particularly, there is a need for a wafer clamping mechanism for a robot that can secure a wafer with sufficient force to prevent slippage and wafer damage during rapid rotation and radial movement. It would be desirable if the clamping mechanism would cause minimal or no particle generation and wafer damage. It would be further desirable if the clamp would automatically engage the wafer except during full extension of the wafer blade when the blade is delivering or picking up a wafer.